1. Field of the Invention
The present invention relates to acoustic wave filter devices that are used as, for example, bandpass filters for mobile telephones, and, more particularly, to an acoustic wave filter device having a ladder circuit configuration.
2. Description of the Related Art
Surface acoustic wave filter devices having a ladder circuit configuration are widely used as bandpass filters for mobile telephones. For example, Japanese Unexamined Patent Application Publication No. 2004-343168 discloses a filter device having a circuit configuration illustrated in FIG. 14. A filter device 101 has a ladder circuit configuration. In a series arm connecting an input terminal 102 and an output terminal 103 to each other, a plurality of series arm resonators 111 to 113 are connected in series with each other. A plurality of parallel arm resonators 114 to 117 are connected to the series arm so as to form parallel arms.
A capacitor 118 is connected in parallel to the parallel arm resonator 114, so that the effective electromechanical coupling coefficient of the parallel arm resonator 114 is lower than that of the parallel arm resonators 115 to 117. As a result, a filter characteristic is abruptly changed at the end portion of a passband.
WO2004/070946 discloses the relationship between a propagation direction ψ of a piezoelectric monocrystal substrate of an acoustic wave filter device and an electromechanical coupling coefficient k2 thereof. The acoustic wave filter device has a configuration in which an Au electrode is formed on an LiNbO3 substrate with Euler angles (0°, 105°, ψ) and an SiO2 film is further formed on the surface of the LiNbO3 substrate. A method of freely changing the value of the electromechanical coupling coefficient k2 within a range of 16% to 0% by changing the value of the propagation direction ψ is disclosed. That is, when the value of the propagation direction ψ is increased, the electromechanical coupling coefficient is monotonically reduced.
In bandpass filters, the amount of attenuation is preferably small in a passband and large outside the passband and the steepness of a filter characteristic is preferably high at boundaries therebetween. In the filter device 101 disclosed in Japanese Unexamined Patent Application Publication No. 2004-343168, in order to reduce the value of the electromechanical coupling coefficient of the parallel arm resonator 114, the capacitor 118 is connected in parallel to the parallel arm resonator 114. As a result, the steepness of a filter characteristic is increased.
In the filter device 101 disclosed in Japanese Unexamined Patent Application Publication No. 2004-343168, it is possible to increase the steepness of a filter characteristic. However, since the anti-resonant frequency of the parallel arm resonator 114 is located at an end portion of the passband, a loss is increased in the passband when the steepness of a filter characteristic is increased on a lower-frequency side of the passband.